Flame-retarding of textiles



FLAME-RETARDING or 'mxmrs Julius Jackson, Westtield, and Alfred Siegel, Summit,

N. 1., aslgnors to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Application December 1 1955 Serial No. 550,467

14 Claims. (Cl. 106-15) This invention relates to new textile fiber and plastic compositions which are resistant to burning. It relates particularly to flame resistant cellulose acetate compositions and to new compositions of matter which impart flame resistance to cellulose acetate and similar materials.

Many attempts have been made to treat ordinary flammable textile materials, and cellulose acetate in particular, in such amanner as to reduce their tendency to burn freely when ignited, or to inhibit their ignition altogether. There is extensive published art in this field,

and attention is called. in particular to Flameproofing.

of Textile Fibers by Little, Reinhold Publishing Company, 1947.

Many inorganic salts will reduce or inhibit the buming of textiles if applied in sufficient quantity, but the required quantity is usually sufficient to cause the materials to become stiff and boardy. Furthermore, such salts are readily removed by washing, and this type of treatment has had limited application and is of little value where repeated laundering may be contemplated. When applied particularly to cellulose acetate fibers which might be used in rugs, most of these prior art methods have caused an undesirable increase in weight, a loss of resilience in the finished rug, a serious loss in the ease of dyeing the fabric or any combination of these deficiencies. Also, the lack of fastness to washing inherent in many of these proposed treatments has limited their application in floor covering materials which are frequently cleaned by scrubbing with aqueous detergents.

It is an object of this invention to produce synthetic textile fibers and plastic compositions, and particularly cellulose acetate compositions, which are resistant to burning, have suffered no loss in tensile strength, exhibit satisfactory hand and resiliency, and which have softening or melting points which are not significantly different from those of the untreated compositions.

A further object is the preparation of certain new flame-retarding compositions particularly adapted to impart these desirable properties to cellulose acetate and like compositions.

These and other objects of this invention are accomplished by incorporating particular metal pyrophosphate compositions in solutions of cellulose acetate or other similar materials and spinning fibers or casting I films therefrom. Iron pyrophosphate and titanium pyrosulting dispersion is added to a solution of cellulosev acetate in an amount to give at least about 0.5% and not more than about 5% of pyrophosphate by weight Patented Mar. 3, 1959 based on the'dry cellulose acetate used. This results in a spinning solution containing the pyrophosphate in suspension, and this composition may then be spun in a conventional manner to give fibers which are highly resistant to burning. Other metallic pyrophosphate compositions showing similar properties include copper pyrophosphate, zirconium pyrophosphate and titanium pyrophosphate.

The following examples illustrate this invention in more detail. Unless otherwise specified, all parts are by weight.

EXAMPLE I 280 parts of ferrous sulfate (FeSO .7H O) is dissolved in about 1,000 parts of water. To this solution are added in turn, a solution of 49 parts of sulfuric acid in about parts of water, and a solution of 19 parts of sodium chlorate in parts of water. The mixture is heated to about 82 0., held at this temperature for about 30 minutes and finally diluted to a volume equivalent to about 3,000 parts at about 35 C. A separately prepared solution of tetrasodium pyrophosphate containing 190 parts of Na P 0 in 2500 parts of water at about 35 C. is added to the iron solution over a period of about 40 minutes. The resulting slurry is practically white and has a pH of about 1.9 to 2.3. It is heated to 82 C. and held at this temperature for about one hour. After stirring for an additional 15 minutes, the precipitate is isolated by filtration, washed substantially free of sulfate ion, dried at about 60 C. and pulverized to give about 250 parts of a fine powder which is almost free of color.

EXAMPLE n 133 parts (0.5 mol) of tetrasodium pyrophosphate (Na P O- is dissolved in about 2,100 parts of water at about 45 C. This solution is then added, in a period of about 40 minutes, to a solution of parts (1 mol) of titanyl sulfate (TiOSO in about 4,000 parts of water at about 32 C. After a brief stirring period, a solution of 67 parts (0.61 mol) of sodium carbonate in 400 parts of water at about 35 C. is added to give a pH of 1.0 to 1.2. If necessary, the pH is brought within this range by adding more or less sodium carbonate as indicated. The slurryis then heated rapidly to about 82 C. and stirred at this temperature for 5-10 minutes. It is cooled by adding a large volume of water, filtered, washed free of sulfate ions, dried at 60 C. and pulverized to give about 400 parts of a white powder which is quite transparent and colorless when dispersed in an oily vehicle.

EXAMPLE III In order to use the compositions prepared in Examples I and II, it is necessary that they be dispersed in a very finely divided form in the cellulose acetate solution from which the fibers or the thin plastic films are formed. One convenient method of obtaining this finely divided form is to disperse the pyrophosphate in a concentrated form in a solution of cellulose acetate by grinding in a ball mill. To avoid contamination with any metallic substance, it is desirable to make such a dispersion in a porcelain lined ball mill using grinding balls of some ceramic composition, such as porcelain. A convenient grinding formula comprises parts of the pyrophosphate, 300 parts of a 16% solution of cellulose acetate in acetone and 720 parts of acetone in a mill of such a size that it is not more than about two-thirds full when the complete charge is in it. This charge is ground for about 72 hours under conventional ball milling conditions, and then it is discharged from the mill and separated from the grinding balls. The resulting composition contains 15% pyrophosphate and 4% cellulose acetate in a slightly viscous suspension, and it is ideally suited for mixing with additional quantifies of cellulose acetate solution to make compositions from which either cellulose acetate fibers or thin plastic films may be formed.

To obtain thin plastic films of cellulose acetate polymer containing the metallic pyrophosphates of this invention, one may mix 6.7 parts of the 15% solution as prepared in the above paragraph with 148 parts of a 16% solution of cellulose acetate to give a composition which contains 4% of pyrophosphate based on the dry cellulose acetate. This composition should be thoroughly mixed for at least one hour with adequate protection from loss of solvent and a film of about .015 inch (wet film thickness) is cast on a sheet of plate glass and dried 30 to 40 minutes at about 70 C. to evaporate the solvent.

To test these films for flammability, strips measuring about one inch by five inches are cut and suspended with the longer dimension held vertically. A small gas flame, about A to /2 inch long from a fine glass tip, is then played for about two seconds on the lower end of the film. Observations may be made with respect to:

A. Ease of ignition B. Ease of propagation or lack of propagation of flame C. Relative amount of volatile combustible gases evolved D. Degree of softening of polymer during burning E. Tendency to after-glow in residue.

The following table summarizes the first two of these properties with respect to compositions made from the flame-retarding compositions of Examples I and II:

In the manufacture of cellulose acetate fibers containing metallic pyrophosphates according to the disclosure of this invention, it is preferred to make such fibers containing approximately 2% of the pyrophosphate based on the weight of the finished fiber. Dispersions of the flame-retarding compositions of Examples I and II prepared according to the disclosures of Example III comprise suitable starting materials to be incorporated with additional solution of cellulose acetate to provide the spinning solutions from which the fibers are spun.

As illustrative of a suitable spinning solution, one may dissolve 700 parts of a suitable grade of cellulose acetate in 2,200 parts of acetone and, when solution is complete, one may add 100 parts of the dispersion of Example Ill. The resulting composition, which comprises about 24% of cellulose acetate, is filtered according to standard procedure and then pumped to the spinnerets so arranged as to give any desired type and size of fiber. The proportions of ingredients listed yield a fiber which contains approximately 2% of metallic pyrophosphate based on the weight of the final dry fiber. The fibers are then used as a deep pile in woven rugs which may be tested for inflammability. In one method of making sucha test for flammability, a small time is applied for seconds to the lower end of a strip of rug held at a 45% angle to the horizontal as in the AATCC test (see Little, Flameproofing of Textile Fibers," page 106), and the flame is then withdrawn and the behavior of the fabric observed as to:

A. Whether ignition took place B. Whether the flame continued after the withdrawal of the igniting flame Accompanying Table 2 shows the results of such a test on fabrics containing 2% of the flame retardants of Examples I and 11 before they had been washed, and Table 3 summarizes a similar series of tests on the same fabrics In preparing the pyrophosphate flame-retarding agents shown to be efiective in this invention, any water soluble pyrophosphate may be used as the source of the pyrophosphate ion. The most readily available salt of this nature is tetrasodium pyrophosphate (Nafl o andthis is the preferred salt for economic reasons only. Potassium pyrophosphate and the corresponding acid salts, such as mango, or KgHgPgOq could be used with equal facility.

In preparing iron pyrophosphate any convenient soluble ferric salt may be used. Probably the most readily available salt of iron is ferrous sulfate obtained in the form of crystalline copper as a lay-product in many commercial operations, particularly in the preparation of titanium compounds from ilmenite. It is convenient to oxidize a solution of ferrous sulfate with sodium chlorate in acid solution as shown in Example I to obtain the desired ferric salt, but this is in no way a limitation on the process. Other common ferric salts may be used with equal facility.

For the preparation of the titanium pyrophosphate of Example 11, any water soluble titanium salt may be used. However, the most practical salt for such purpose is titanyl sulfate (Ti0SO This is a common intermediate compound in the preparation of titanium compounds from its ores, and it also may be readily prepared by dissolving a hydrous titanium oxide in excess sulfuric acid. This salt is only soluble in solutions containing substantial excess of acid, and it is necessary to introduce a neutralization step whenever the salt is used. The corresponding titanium oxychloride (Ti0Cl,) could also be used, and when making zirconium pyrophosphate, it is the zirconium oxychloride which is the most readily available and preferred salt for use.

In the preparation of iron pyrophosphate, the examples show the use of .714 mol of pyrophosphate instead of the theoretical 0.75 mol per mol of iron. Products have been made in which this ratio has varied from as low as about 0.6 mol of pyrophosphate up to about 1 mol of pyrophosphate per mol of iron. This is a range of from 1 to 1.6 mols of iron per mol of pyrophosphaie. The water content is from 25-30.8%. As the amount of pyrophosphate is increased, the amount of sodium in the resulting compound increases and the yield also increases, but there is little change in the properties of the cellulose acetate compositions made therefrom. These results may be explained on the assumption that a solid phase containing sodium pyrophosphate is being formed. and that this phase is not increasing in solubility with increased sodium content. A typical analysis of the solid phase isolated at the preferred ratio of ingredients is:

This analysis is consistent with the presumption of the formation of solid solutions of ferric pyrophosphate and tetrasodium pyrophosphate. It is also pointed out that this composition may vary over a considerable range while retaining the effectiveness desired. A preferred range for the compositions is: iron as mo, 29 to 31%; phosphorus as P 40 to 42%; sodium 0.2 to 2% and the balance being essentially water. In addition to the above ingredients the compositions will contain very minor amounts of impurities but this is to be expected with any composition which is not subjected to special purification means.

Titanium pyrophosphate which is another preferred flame retardant may be obtained by coinbining a titanium salt and tetrasodium pyrophosphate in a 2 to 1 molar ratio, while maintaining the pH below about 2.5. The

- preferred pH is 1.0 to 1.5. With the proper maintenance of a low pH, the titanium ion may be decreased to about 1.5 mols per mol of pyrophosphate, and increased to as much as 2.5 mols without markedly affecting the properties of the cellulose acetate compositions made therefrom. It is believed that compositions containing significantly more than about 2 mols of titanium per mol of pyrophosphate probably contain some hydrous titanium oxide, and they tend to develop a certain amount of opacity in the resulting cellulose acetate compositions. Nevertheless, the fire retardant properties are retained. The analysis of a typical titanium pyrophosphate composition made according to the preferred process of Example II is:

This approximates a ratio of 2 mols TiO, to 1 mol P 0 or an empirical formula of (TiO), P 0 with water of hydration. Other products made by slight variations of this preferred process have given analyses in which the ratios of TiO, to P 0; vary from about 1.5 to about 2.5, with a water content of about 20% and it is contemplated that compounds within this range may be used to obtain the advantages of this invention. It will be found that a zirconium pyrophosphate made in the same manner will have comparable properties. Titanium and zirconium pyrophosphates with the above-described compositions are considered to be new compositions of matter.

In the preparation of cellulose acetate compositions from the flame-retarding agents described in this invention, it is necessary that these agents be dispersed in extremely small particle sizes before they may be used. This requirement of extreme dispersion is particularly important in the preparation of fibers for several reasons. Particles of any appreciable particle size might either clog the filter before the spinneret or clog the holes in the spinnerets themselves; or if it did pass through the spinneret, a large particle might weaken the very fine filaments and cause breaks during their subsequent processing. Also, particles of any significant size may impair the appearance of either fibers or films. Consequently, it is necessary to introduce a step which disperses the flame-retarding compositions in the spinning solution, but the particular method by which this is done is not in any way a part of this invention. It is very convenient to use techniques generally applicable to the dispersion of pigments in liquid compositions such as grinding in a ball mill in a fluid medium, and the process of Example III in which the fluid medium is a solution of a small amount of cellulose acetate in an appropriate solvent, such as acetone, is merely typical of such processes. Dispersion can be effected in the solvent alone, but experience has shown that it is more effectively done in the presence of a small amount of cellulose acetate. Other dispersing agents could be used if one desires them in the subsequent spinning solutions. The amount of solids in this dispersion preparation is not important, except that it must be low enough to maintain a suitable consistency for effective ball milling. Other effective methods of dispersion in such liquid compositions include grinding on roller mills, passing through homogenizers, and plastic milling in compositions of high viscosity such as cellulose acetate containing only a small amount of solvent, or even molten cellulose acetate.

The preparation of cellulose acetate films and fibers 1s a very complex art in which the exact composition of the solutions from which the films are cast or the fibers are spun may be varied widely in a manner well known in the art to obtain particular properties in the resulting films or fibers. The only point in these possible variatrons which is significant to this invention is the introductron of the preferred pyrophosphates to confer upon the compositions fire retardant properties. In the introduction of these pyrophosphates, it is convenient, as shown in Examples 111 and IV, to add a desired amount of a dispersron prepared as in Example HI to a composition suitable for the ultimate desired purpose in such amounts as will give from about 0.5% to about 5% of the pyrophosphate on the basis of the final cellulose acetate composition. In the case of the preparation of cellulose acetate fibers, it is preferred to add this separately prepared dispersion of the flame-retarding agent to the spinning solution just prior to its filtration before spinning. A significant improvement in fire retardant properties of cellulose acetate fibers is obtained with as little as 0.5% of the pyrophosphate, and very highly desirable products are obtained in the range of 1 to 2%. Fibers made in this way show substantially no change in properties when compared to fibers containing no pyrophosphate, except that a very slight yellow coloration is obtained when iron pyrophosphate is used. The fibers can be further dyed, or colored pigments may be introduced into the spinning solution to obtain any desired color. Itis not intended to exclude the possibility of using amounts of pyrophosphate above the preferred maximum of 5%, but the introduction of such larger amounts may result in an alteration of the properties of the fiber, and it is also economically unattractive.

This invention has been set forth as having its principal utility in imparting flame resistance to "cellulose acetate by which term is meant the acetone soluble polymer commonly obtained by partially hydrolyzing cellulose triacetate so that it contains 2.3 to 2.5 acetate groups per structural unit in the cellulose chain. This is the most widely used modification, but it is contemplated that true cellulose triacetate which can be spun from chloroform solu tion, for instance, can be made flame resistant in a comparable manner. A number of other cellulose esters and ethers have been used for special effects, such as cellulose butyrate or cellulose propionate, as well as mixed esters containing both acetate and butyrate or propionate groups. These are also formed into fibers or films from acetone solutions, and they can be made flame resistant in the manner set forth above. The cellulose others such asethyl cellulose and benzyl cellulose may also be formed into films from solvent solutions, and flame resistance may be imparted by incorporating the metal pyrophosphates of this invention.

Cellulose acetate fibers made according to the disclosures of this invention offer many advantages with respect to freedom from flammability over prior art cellulose acetate fibers. For the first time, cellulose acetate I fibers have been given a flame resistant treatment which I i I does not significantly alter the other-propertiesof the J fibers. I There isno significant lowering :of the softening point; there is no significant change in the resilience or I I hand of the textiles made therefrom; and there is noloss I in the ability to dyethe textilese In particular, it is believed that these products open for the first time the wide i I I I scale use of cellulose acetate in themanufacture of rug I fibers. I Such products are also potentiallyusefirl in many otherap'plications of cellulose acetate such as in knit I goods or the like, and these fire resistant products would otter marked advantages over: products I currently. availl able.

We claim:

I I l. A fire retardant celluloseeomposition consistingesl sentiall'y of a major amount of a cellulose material se I I I I I lectediromthe. group consisting of cellulose ethersfland I cellulose esters and a minor amount of a metal gym! I phosphate sufficient to impart flame retardant properties I I to said cellulose material; said metal pyrophosphatej being selected 4mm the I group consisting of I iron, cooper, titanium, and zirconium pyrophosphates. f I 2. A fire retardant cellulose composition consisting I essentially of a major amount of a cellulose material se-.

1 I cellulose ester-sand from 0.5 to 5% of a metal pyrophos- I I phate selected 'fromthe'g'roup consistingof imnmop ri lected from the group consisting of cellulosejethersand titanium,and zirconiumpyrophosphatesg I I 1 I 3, A fire retardantjcellulose' acetate composition con 3 I j sisting essentially of a major amount of cellulose acetate 1 I and from 0.5 to 5% of iron pyrophosphate; t I

. I 4. A fire retardant cellulose acetate composition con-. sisting essentially. of a majoramount of cellulose acetate and from 0.510 5% of titanium pyrophosphate.

I I I 5. A fire retardant cellulose acetate composition con- I sisting essentially of a major-amount of; cellulose acetate and from 0.5 to 5 of zirconium :pyrophosphate; I

I I I I 6.. A- fire retardant cellulose acetate composition consisting essentially of a majoramount of cellulose acetate and from 0.55% of a titanium pyrophosphate in which the molar ratio of titanium to pyrophosphate radical ranges from 1.5-2.5:1 and in which the water content is about 7. A fire retardant cellulose acetate composition consisting essentially of a major amount of a cellulose acetate and from 0.5-5% of a zirconium pyrophosphate in which the molar ratio of n'rconium to pyrophosphate radical ranges from 1.5-2.5 :1 and in which the water content is about 20%.

' 8; A fire retardant cellulose acetate composition com I sisting essentially of a major amount of cellulose acetate 1 and from 0.54% of an iron pyrophosphate in which the t I molar ratio: of iron :to pyrophosphate radical ranges from 1-1.6:1 and in which the water content rangesfrom I 'SLA fire retardant celluloseacctatecomposition con- I essentially of a major amount of cellulose acetate I 1 and from 0.5 to 5% of iron pyrophosphatehaving an I analysis of from about. 29 319?) iron as Fe o -42% phosphorus as P 05, and from about 0.2 2% sodium, g

and the balance being essentially water: I I

I I 10. A:fire:retardant.cellulose acetate composition consisting essentially of a major amount :of cellulose acetate: I and from'0.5 to 5% of titanium pyrophosphate having an e xanalysis of; about 42% as TiO 36% phosphorusas P 0 ,andabout20% water;

:11; A flame-retarding composition forcellulose mate- I I rials selected fromthe group consisting of cellulose ethers and cellulose esters consisting. essentially of a, hydrous; I utanium pyrophosphate, .in which the molar ratio of I titanium to pyrophosphate radical may vary from 1.5-

'i 2.5mm inwhichthe watercontentis about:20%.l

12; The composition of claim ll in which themolar ratio of to pyrophosphate is about 2:1. I

I a 13,. A flame-retardingcompositionforcellulose mate-: I I rialsselected from the group consisting of cellulose ethers I Rand; cellulose esters consisting'essentially of a-hydrous I zirconium pyrophosphate in whichthemolar ratio :of zit I I conium to pyrophosphate radical may vary from 1.5- 2.5.:1 and inwhich the watercontent is about 20%. I i I I f 14; The composition of claim l3iiuwhichithe molar i ratio of zirconium topyrophosphate isabout 2:1.

Rosenheim: Z. Anorg. Allgem. Chem., vol. 153 (1926), pp. 126-142.

keiercncelcitedinthefileofthispatent::.

UNITED STATES PATENT OFFICE CERTIFICATE OF CQR i ECTION Patent No, 2,876,117 March 3, 1959 Julius Jackson et a1.

It is hereby certified that error appears in the-printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4, line 39, for "copper' read copperas Signed and sealed this 23rd day of June 1959.

( SEAL) Attest:

KARL H. AXLINE ROBERT C. WATSON Attesting Oflicer Commissioner of Patents 

1. A FIRE RETARDANT CELLULOSE COMPOSITION CONSISTING ESSENTIALLY OF A MAJOR AMOUNT OF A CELLOUSE MATERIAL SELECTED FROM THE GROUP CONSISTING OF CELLOUSE ETHERS AND CELLOUSE ESTERS AND A MINOR AMOUNT OF A METAL PYROPHOSPATE SUFFICIENT TO IMPART FLAME RETARDANT PROPERTIES TO SAID CELLOUSE MATERIAL, SAID METAL PYROPHOSPHATE BEING SELECTED FROM THE GROUP CONSISTING OF IRON, COOPER, TITANIUM, AND ZIRCONIUM PYROPHOSPHATES.
 11. A FLAME-RETARDING COMPOSITION FOR CELLLOUSE MATERIALS SELECTED FROM THE GROUP CONSISTING OF CELLOUSE ETHERS AND CELLOUSE ESTERS CONSISTING ESSENTIALLY OF A HYDROUS TITANIUM PYROPHOSPHATE IN WHICH THE MOLAR RATIO OF TITANIUM TO PYROPHOSPHATE RADICAL MAY VARY FROM 1.5295:1 AND IN WHICH THE WATER CONTENT IS ABOUT 20%. 